Code Division Multiple Access (CDMA) has become an important component of recent communication systems. CDMA is a channel access communication scheme used by various radio communication technologies, including cellular networks. CDMA allows several transmitters to send information simultaneously over a single communication channel, that is, Multiple Access. This lets several users share a band of frequencies. A CDMA scheme typically employs spread-spectrum methods and a special coding scheme, in which each transmitter is assigned a code, to allow multiple users to be multiplexed over the same physical channel. Typically, in CDMA, the modulated coded signal has a much higher data bandwidth than the data being communicated. A spread spectrum scheme uniformly spreads the bandwidth of the data for the same transmitted power.
Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. When there is good separation between the signal of a desired user and the signals of other users, a CDMA system performs well. Generally, CDMA can be synchronous (orthogonal codes); or asynchronous (pseudorandom codes).
In CDMA channel access, a chip is a pulse of a Direct-Sequence Spread Spectrum (DSSS) code, such as a pseudo-noise code sequence used. In a binary direct-sequence system, each chip is typically a rectangular pulse of +1 or −1 amplitude, which is multiplied by a data sequence (similarly +1 or −1 representing the message bits) and by a carrier waveform to make the transmitted signal. The chips are therefore just the bit sequence out of the code generator. The chip rate of a code is the number of pulses per second (chips per second) at which the code is transmitted/received. The chip rate is larger than the symbol rate, meaning that one symbol is represented by multiple chips. In under loaded or fully loaded CDMA systems the number of chips is greater than or equal to the number of users. On the other hand, in overloaded CDMA the number of chips is less than the number of users. Thus, overloaded systems are more efficient from bandwidth point of view.
Under loaded or fully loaded synchronous CDMA exploits mathematical properties of orthogonality between vectors representing the data strings. Each user in these systems uses a code orthogonal to the others' codes to modulate their signal. Orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other.
However, in overloaded CDMA there are no such orthogonal codes and so it is impossible to take the advantages of this concept in user data extraction. In fact, in these cases, encoders and decoders are typically complex and require numerous multiplication and addition operations.